State-of-the-art ultrafast fiber lasers currently are limited in peak power by excessive nonlinearity and in average power by modal instabilities. Coherent beam combination in space and time is a successful strategy to continue power scaling by circumventing these limitations. Following this approach, we demonstrate an ultrafast fiber-laser system featuring spatial beam combination of 8 amplifier channels and temporal combination of a burst comprising 4 pulses. Active phase stabilization of this 10-armed interferometer is achieved using LOCSET and Hänsch-Couillaud techniques. The system delivers 1 kW average power at 1 mJ pulse energy, being limited by pump power, and delivers 12 mJ pulse energy at 700 W average power, being limited by optically induced damage. The system efficiency is 91% and 78%, respectively, which is due to inequalities of nonlinearity between the amplifier channels and to inequality of power and nonlinearity between the pulses within the burst. In all cases, the pulse duration is ~260 fs and the M2-value is better than 1.2. Further power scaling is possible using more amplifier channels and longer pulse bursts.

We report multi-mJ energy (>5mJ) extraction from femtosecond-pulse Yb-doped fiber CPA using coherent pulse stacking amplification (CPSA) technique. This high energy extraction has been enabled by amplifying 10’s of nanosecond long pulse sequence, and by using 85-µm core Yb-doped CCC fiber based power amplification stage. The CPSA system consists of 1-GHz repetition rate mode-locked fiber oscillator, followed by a pair of fast phase and amplitude electro-optic modulators, a diffraction-grating based pulse stretcher, a fiber amplifier chain, a GTI-cavity based pulse stacker, and a diffraction grating pulse compressor. Electro-optic modulators are used to carve out from the 1-GHz mode-locked pulse train an amplitude and phase modulated pulse burst, which after stretching and amplification, becomes equal-amplitude pulse burst consisting of 27 stretched pulses, each approximately 1-ns long. Initial pulse-burst shaping accounts for the strong amplifier saturation effects, so that it is compensated at the power amplifier output. This 27-pulse burst is then coherently stacked into a single pulse using a multiplexed sequence of 5 GTI cavities. The compact-footprint 4+1 multiplexed pulse stacker consists of 4 cavities having rountrip of 1 ns, and one Herriott-cell folded cavity - with 9ns roundtrip. After stacking, stretched pulses are compressed down to the bandwidth-limited ~300 fs duration using a standard diffraction-grating pulse compressor.

The Laser megajoule (LMJ) is a French large scale laser facility dedicated to inertial fusion research. Its front-ends are based on fiber laser technology and generate highly controlled beams in the nanojoule range. Scaling the energy of those fiber seeders to the millijoule range is a way explored to upgrade LMJ’s architecture.
We report on a fully integrated narrow line-width all-fiber MOPA prototype at 1053 nm designed to meet stringent requirements of large-scale laser facilities seeding. We achieve 750 µJ temporally-shaped pulses of few nanoseconds at 1 kHz. Thanks to its original longitudinal geometry and its wide output core (26µm MFD), the Yb-doped tapered fiber used in the power amplifier stage ensures a single-mode operation and negligible spectro-temporal distortions. The transport of 30 kW peak power pulses (from tapered fiber) in a 17 m long large mode area (39µm) hollow-core (HC) fiber is presented and points out frequency modulation to amplitude modulation conversion management issues. A S² measurement of this fiber allows to attribute this conversion to a slightly multimode behavior (< 13dB of extinction between the fundamental mode and higher order modes). Other HC fibers exhibiting a really single-mode behavior (<20 dB) have been tested and the comparison will be presented in the conference. Finally, fiber spatial beam shaping from coherent Gaussian beam to coherent top-hat intensity profile beam in the mJ range with a specifically designed and fabricated fiber will also be presented.

Temporal coherent beam combining of pulsed fiber lasers has gained a lot of interest as it could pave the way for fiber lasers to compete with bulk lasers in terms of pulse energy. We purpose to stack up the symmetric output of an oscillator by phase and polarization switching. This way we can avoid the effects of gain dynamics and are able to utilize all the seed power as opposed to regular pulse picking approaches.

We used a 47 MHz femtosecond fiber oscillator centered at 1558 nm (5 nm bandwidth). Feeding the pulse train into a Mach Zehnder interferometer and adding electro optic modulators (EOMs) in both optical paths, we imprint a π phase shift in one path on every other pulse. We then use a polarization beam splitter to combine the two beams and stabilize the resulting polarization with a unity gain significantly lower than MHz. Therefore after the two beams are combined, the π phase shift stays intact and neighboring pulses have their polarization flipped. In a following PBS horizontal and vertically polarized pulses are separated. The vertical polarized pulses are delayed and combined with the next pulse. This halves the repetition rate basically acting like a pulse picker but without discarding 50% of the output power. The efficiency of the experiment was so far limited by linear losses in the PBS and linear losses due to the pickups for the photodiodes. We will give an update un the latest results and our plans to go forward.

In this contribution, we present a spatio-temporal coherent beam combining setup in a proof-of-principle experiment with an entirely fiber-coupled front-end. Unlike in previous experiments, where the temporal pulse division was achieved using free-space optical delay lines, the pulses are taken directly from the pulse train of the oscillator. Thereby, the free-space paths and the alignment requirement are cut in half. The combination inevitably remains in free-space considering application in high-power lasers. For the combination of 4 temporally separated pulses, a combining efficiency larger than 95% is demonstrated. The efficiency is largely independent of the combined pulse energy and temporal contrasts close to the theoretically estimated maximum are reached. Potentially, this approach allows for self-optimization of the combination due to the many degrees of freedom accessible with the electro-optic modulators.

Chirped pulse monolithic fiber amplifier based on a newly developed tapered polarization maintaining Yb-doped fiber has been developed and optimized. A novel amplification regime in a relatively long (220 cm) tapered fiber of improved design, which has been theoretically predicted, allowed us to achieve an ultimate high peak power. In this regime, the signal propagates most of the fiber without amplification and growths very rapidly only in the last 80 cm of the tapered fiber, which has a mode field area of approximately 1000 μm2 near the output. We have demonstrated amplification of 20 ps chirped pulses centered at 1056-nm with spectral width of 20 nm to 0.7 MW peak power directly from the tapered fiber amplifier. The pulses had a diffraction limited quality (M2 ~ 1.124) and could be compressed down to 350 fs with 50% efficiency. In addition, amplification of narrow-band 9 ps pulses centered at 1064 nm to a peak power of 1.8 MW directly from the tapered fiber amplifier was demonstrated.

An all-fiber ultrafast dissipative soliton laser at 1.3 microns based on phoshosilicate fiber doped with bismuth is presented. A nonlinear optical loop mirror containing high-germanium fiber with high nonlinearity and large positive dispersion was used. The scheme yields 11.3 ps pulses with energy of 1.7 nJ at repetition rate of 3.5 MHz. By means of bismuth-doped fiber amplifier and diffraction gratings compressor, the pulses were amplified up to 8.5nJ and compressed down to 530 fs. To achieve best results the optimal bismuth active fiber was chosen according to the investigated dependence of the gain coefficient on bismuth active centers concentration in phosphosilicate fibers.

Thulium-doped fiber lasers are an attractive concept for the generation of mid-infrared (mid-IR) ultrashort pulses around 2 μm wavelength with an unprecedented average power. To date, these systems deliver >150 W of average power and GW-class pulse peak powers with output pulse durations of a few hundreds of fs. As some applications can greatly benefit from even shorter pulse durations, the spectral broadening and subsequent temporal pulse compression can be a key enabling technology for high average power few-cycle laser sources around 2 μm wavelength. In this contribution we demonstrate the nonlinear compression of ultrashort pulses from a high repetition rate Tm-doped fiber laser using a nitrogen gas-filled hollow capillary. Pulses with 4 GW peak power, 46 fs FWHM duration at an average power of 15.4 W have been achieved. This is, to the best of our knowledge, the first 2 μm laser delivering intense, GW-pulses with sub 50-fs pulse duration and an average power of >10 W. Based on this result, we discuss the next steps towards a 100 W-level, GW-class few-cycle mid-IR laser.

Experimental demonstrations of Tm-doped fiber amplifiers (typically in CW- or narrow-band pulsed operation) span a wavelength range going from about 1700 nm to well beyond 2000 nm. Thus, it should be possible to obtain a bandwidth of more than 100 nm, which would enable sub-100 fs pulse duration in an efficient, linear amplification scheme. In fact, this would allow the emission of pulses with less than 20 optical cycles directly from a Tm-doped fiber system, something that seems to be extremely challenging for other dopants in a fused silica fiber. In this contribution, we summarize the current development of our Thulium-doped fiber CPA system, demonstrate preliminary experiments for further scaling and discuss important design factors for the next steps. The current single-channel laser system presented herein delivers a pulse-peak power of 2 GW and a nearly transform-limited pulse duration of 200 fs in combination with 28.7 W of average power. Special care has been taken to reduce the detrimental impact of water vapor absorption by placing the whole system in a dry atmosphere housing (<0.1% rel. humidity) and by using a sufficiently long wavelength (1920-1980 nm). The utilization of a low-pressure chamber in the future will allow for the extension of the amplification bandwidth. Preliminary experiments demonstrating a broader amplification bandwidth that supports almost 100 fs pulse duration and average power scaling to < 100W have already been performed. Based on these results, a Tm-doped fiber CPA with sub-100 fs pulse duration, multi-GW pulse peak power and >100 W average power can be expected in the near future.

Ultrashort-pulse laser systems are an enabling technology for numerous applications. The stability of such systems is especially crucial for frequency metrology and high precision spectroscopy. Thulium-based fiber lasers are an ideal starting point as a reliable and yet powerful source for the nonlinear conversion towards the mid-IR region. Recently, we have demonstrated that nonlinear self-compression in a fused silica solid-core fiber allows for few-cycle pulse duration with up to 24 MW peak power using a high-repetition rate thulium-based fiber laser system operating at around 2 μm wavelength [1]. This experiment operates near the self-focusing limit of about 24 MW for circular polarization, which increases the requirements for the system stability due to the risk of a fiber damage. Here, we present a self-protecting nonlinear compression regime allowing for long-term operation and high output-pulse stability with very similar output performance.

One of the current challenges towards the development of ultrafast 2 microns all-fiber laser systems delivering transform-limited pulses is to manage dispersion and nonlinearities which are well-known limiting factors in fiber-based systems due to their negative impact on pulse duration and shape.
Here, we present what we believe to be, to the best of our knowledge, the first all-solid step-index dispersion tailored fiber designed with anomalous dispersion around 2 microns. This all-solid, step-index ultra-high numerical aperture (UHNA) fiber offers an efficient and simple alternative compared to existing approaches such as free-space optical systems or micro-structured fibers that are complex to manufacture and handle. The combination of highly Ge-doped core with a small core diameter allows tailoring the material and waveguide components of the dispersion to reach the anomalous dispersion required by the application.
In this work, details will be provided using experimental and calculated values via the example of a non-PM UHNA fiber with 2.45 microns core and 0.34 NA. This fiber was designed to achieve anomalous dispersion of -45 ps/(nm.km) at 2 microns. It will be shown that the UHNA fiber design can be further tuned to achieve specific values of anomalous dispersion and dispersion slope. The fiber performances were confirmed using a 2 microns chirp-pulsed fiber amplifier where the pulse duration was measured at 24 ps and 4.3 ps without and with the UHNA fiber respectively. A PM-UHNA fiber design is currently being developed and will be characterized and tested following a similar fashion.

We present development of a nanosecond Q-switched Tm3+-doped fiber laser with 16 W average power and 4.4 kW peak power operating at 1940 nm. The laser has a master oscillator power amplifier design, and uses large mode area Tm3+-doped fibers as the gain medium. Special techniques are used to splice Tm3+-doped fibers to minimize splice loss. The laser design is optimized to reduce non-linear effects, including modulation instability. Pulse width broadening due to high gain is observed and studied in detail. Medical surgery is a field of application where this laser may be able to improve clinical practice. The laser together with scanning galvanometer mirrors is used to cut precisely around small footprint vessels in tissue phantoms without leaving any visible residual thermal damage. These experiments provide proof-of-principle that this laser has promising potential in the laser surgery application space.

A careful comparison of experiment and theory is important both for basic research and systematic engineering design of Thulium fiber amplifiers operating in the 2 μm region for applications such as LIDAR or spectroscopy (e.g. CO2 atmospheric absorption at 2051.4 nm). In this paper we report the design and performance of a multistage high-power PM Tm-doped fiber amplifier, cladding pumped at 793 nm. The design is the result of a careful comparison of numerical simulation, based on a three level model including ion-ion interactions, and experiment. Our simulation model is based on precise measurements of the cross sections and other parameters for both 6 and 10 μm core diameter fibers. Good agreement for several single and multistage amplifier topologies and operating conditions will be presented. Origins of the difference between theory and experiment are discussed, with emphasis on the accuracy of the cross sections and the cross relaxation parameters. Finally based on our simulation tool, we will demonstrate a design with an output power greater than 10 W for a multistage amplifier with a single-frequency signal at 2050 nm. The power stage was constructed with a 6 μm active fiber showing a 64 % optical slope efficiency. The output power is found to be within 5 % of the simulated results and is limited only by the available launched pump power of ~24 W. No stimulated Brillouin scattering is observed at the highest output power level for an active fiber well thermalized.

Large-mode area (LMA) thulium-doped fibers (TDF) are one of the key components when designing 2μm laser and amplifier systems aiming to further scale deliverable output powers. Current design limitations of LMA TDF’s affecting optical-to-optical efficiency and output beam quality are well-understood. In the present work, design optimizations focused on the core and pedestal waveguides of the active fiber are proposed. Using experimental and numerical tools, the effect of splice-induced heat on the refractive index profile of the active fiber is investigated. We demonstrate that fibers designed with larger pedestal-to-core ratios suffer less index distortions during splicing allowing the end-user to achieve high coupling efficiencies and high beam qualities in a reliable fashion.

We measure changes in the 2um absorption and emission spectra of thulium-doped silica fiber lasers operating from 80 K – 373 K. Reduction of the long wavelength tail of the 3H6-3H4 absorption feature under cryogenic cooling allows for efficient lasing in the 1800nm region. Greater than 17 W of output power was generated at 1850 nm by 793 nm diode-pumping a free-running single-mode thulium oscillator under cryogenic cooling conditions.

This work studies the accumulated nonlinearities when amplifying a narrow linewidth 2053 nm seed in a single mode Tm:fiber amplifier. A <2 MHz linewidth CW diode seed is externally modulated using a fiberized acousto-optic modulator. This enables independent control of repetition rate and pulse duration (>30 ns). The pulses are subsequently amplified and the repetition rate is further reduced using a second acousto-optic modulator. It is well known that spectral degradation occurs in such fibers for peak powers over 100's of watts due to self-phase modulation, four-wave mixing, and stimulated Raman scattering. In addition to enabling a thorough test bed to study such spectral broadening, this system will also enable the investigation of stimulated Brillouin scattering thresholds in the same system. This detailed study of the nonlinearities encountered in 2 μm fiber amplifiers is important in a range of applications from telecommunications to the amplification of ultrashort laser pulses.

A 2-μm linear-polarized single-frequency Brillouin-Thulium fiber laser (BTFL) has been experimentally investigated for linewidth narrowing. The threshold for the Brillouin pump is around 200 mW, and more than 205 mW single-frequency Stokes laser was achieved with the 793 nm pump power of 8.5 W. The linewidth of the fiber laser has been narrowed for ~8 times, from 34 to 4.6 kHz. The measured RIN of the BTFL is <-150 dB/Hz for frequency above 2 MHz, which approaches the shot noise limit.

Mode instabilities in fiber amplifiers are analyzed by a new approach, considering the stability of the steady state FM amplification in the presence of transverse amplitude and/or phase perturbations, taking into account the effects of population inversion and thermal loading due to quantum-defect heating. Population inversion contribution to instability is shown to be dominant at low powers and high inversion. Under high powers and low inversion (high amplifier saturation) the thermal effects dominate the instability behavior. A simple and easy to interpret TMI power threshold formula is derived for the first time.

Transverse mode instabilities (TMI) have become a very serious problem for the further scaling of the average power of fiber laser systems. Recently the strong impact that photodarkening (PD) has on the TMI threshold of Yb-doped fiber laser systems has been revealed. This is a remarkable finding since it opens the door to a significant increase of the average power of fiber laser systems in the near future. The key to achieve this is to reduce the amount of PD losses in the fiber, which can be done with an optimization of the glass composition in the fiber. In this work we perform a theoretical study on the impact that co-dopants such as Al and P have on PD and on the TMI threshold. This analysis tries to find the optimum glass composition from the point of view of TMI. It is shown that in a short rod type fiber, changing the glass composition only leads to a modest increase of the TMI threshold due to the degradation of the cross-sections. This demonstrates that the optimization of the glass cannot be done attending only to the PD losses at the cost of the laser cross-sections. In spite of this, changing the glass composition can bring benefits in pulsed operation in terms of the stored energy. Additionally, other fiber geometries different from the rod-type can benefit in a greater degree by introducing co-dopants in the glass.

Thermally induced transverse modal instabilities (TMI) have attracted these five years an intense research efforts of the entire fiber laser development community, as it represents the current most limiting effect of further power scaling of high power fiber laser. Anyway, since 2014, a few publications point out a new limiting thermal effect: fiber modal degradation (FMD). It is characterized by a power rollover and simultaneous increase of the cladding light at an average power far from the TMI threshold together with a degraded beam which does not exhibit temporal fluctuations, which is one of the main characteristic of TMI.
We report here on the first systemic experimental study of FMD in a high power photonic crystal fiber. We put a particular emphasis on the dependence of its average power threshold on the regime of operation. We experimentally demonstrate that this dependence is intrinsically linked to regime-dependent PD-saturated losses, which are nearly three times higher in CW regime than in short pulse picosecond regime. We make the hypothesis that the existence of these different PD equilibrium states between CW regime and picosecond QCW pulsed regime is due to a partial photo-bleaching of color centers in picosecond regime thanks to a higher probability of multi-photon process induced photobleaching (PB) at high peak power. This hypothesis is corroborated by the demonstration of the reversibility of the FMD induced in CW regime by simply switching the seed CW 1064 nm light by a short pulse, picosecond oscillator.

The phenomenon of transverse mode instabilities (TMI) is currently the most limiting effect for the scaling of the average output power of fiber laser systems with nearly diffraction-limited beam quality. Thus, it is of high interest to develop efficient mitigation strategies to further enhance the performance of fiber laser systems. By actively modulating the pump power of an Yb-doped rod-type fiber amplifier, it was possible to weaken the thermally-induced refractive index grating along the fiber and, thus, to mitigate TMI to a large extent. A significant advantage of this approach is that it can be easily integrated in any existing fiber-laser system since no further optical components are needed. A function generator connected to the pump diode driver was used to achieve the modulation. With this setup we were able to extract a fully stabilized beam at ~ 1.5 times above the TMI threshold. Furthermore, a stabilization of the beam was still feasible at an average output power of 628 W, which is more than three times higher than the free-running TMI threshold of that particular fiber under identical conditions (e.g. seed power). This is the highest average output power reported from a single-channel rod-type fiber amplifier with a high-quality stabilized beam, to the best of our knowledge.

The power scaling of high power fiber lasers has decelerated recently, due to transverse mode instability (TMI) and photodarkening (PD). The origin of TMI is a power transfer from the fundamental mode of the fiber to higher transverse modes due to self-induced thermo-optical long period gratings. The excitation of higher modes can lead to temporal instability and a bend-loss-induced reduction of the laser power.

Over the lifetime of a fiber laser, the TMI threshold is decreased due to photodarkening of the fiber. Many investigations have been made to model both effects, but the microscopic mechanisms both of TMI and PD are not yet fully understood. The existing models are either comprehensive, but very slow and therefore limited to the simulation of short fibers, or reduced models that do not take transverse effects into account. Furthermore, these models have been applied only to single-pass fiber amplifiers so far.

We present a hierarchical numerical approach that allows to first precalculate the transverse distribution of the photodarkening losses, and then apply the precalculated data to a scalar coupled-mode model of the fiber laser. As a result, it is possible to perform virtual long term tests simulating several 10 000 hours of laser operation in a few hours. The transverse distribution of photodarkening losses in the fiber and the mode coupling gain can be analyzed at any cross section along the fiber.

The simulation results are compared to experimental data, which demonstrates the feasibility of the approach to predict the TMI threshold for different laser setups.

The phenomenon of transverse mode instabilities (TMI) is currently the most limiting effect for the scaling of the average output power of fiber laser systems with nearly diffraction-limited beam quality. Even though a significant amount of knowledge on TMI in single-pass fiber amplifiers has been generated in the last years, relatively little is known about this effect in multi-pass amplifiers and oscillators. In this contribution TMI is experimentally investigated in a double-pass fiber amplifier, for the first time to the best of our knowledge. The TMI threshold was found to be significantly lower in the double-pass configuration than in the single-pass arrangement. Furthermore, the investigations unveiled a complex dynamic behavior of the instabilities in the double-pass fiber amplifier.

The development of high-power fiber lasers is of great interest due to the advantages they offer relative to other laser technologies. Currently, the maximum power from a reportedly single-mode fiber amplifier stands at 10 kW. Though impressive, this power level was achieved at the cost of a large spectral linewidth, making the laser unsuitable for coherent or spectral beam combination techniques required to reach power levels necessary for airborne tactical applications. An effective approach in limiting the SBS effect is to insert an electro-optic phase modulator at the low-power end of a master oscillator power amplifier (MOPA) system. As a result, the optical power is spread among spectral sidebands; thus raising the overall SBS threshold of the amplifier. It is the purpose of this work to present a comprehensive numerical scheme that is based on the extended nonlinear Schrodinger equations that allows for accurate analysis of phase modulated fiber amplifier systems in relation to the group velocity dispersion and Kerr nonlinearities and their effect on the coherent beam combining efficiency. As such, we have simulated a high-power MOPA system modulated via filtered pseudo-random bit sequence format for different clock rates and power levels. We show that at clock rates of ≥30 GHz, the combination of GVD and self-phase modulation may lead to a drastic drop in beam combining efficiency at the multi-kW level. Furthermore, we extend our work to study the effect of cross-phase modulation where an amplifier is seeded with two laser sources.

Experimental observation of a novel effect of long-term mode shape degradation in a high peak power ytterbium-doped pulsed fiber lasers based on large mode area step-index fibers has been reported for the first time. It is shown, that the degradation is caused by power coupling from LP01 to LP11 mode, occurring on a long period refractive index grating (LPG) formed in the active fiber core. The photo-darkening process was found to play a significant role in the formation of a LPG.

High-power fiber lasers became important devices in many industrial and health care fields. The key for high-power operation of fiber lasers is the double-clad fiber technology transforming lower-brightness pumps into high-brightness laser beams. Efficient pump absorption in the active core of the double-clad fiber is crucial for reliable and economic operation of high power fiber lasers. In our recent work we extensively studied the dependence of the pump absorption efficiency on bending and twisting of the fiber. For the first time we theoretically predicted and later experimentally demonstrated significant enhancement of pump absorption efficiency by simultaneous bending and twisting of the double-clad fiber.
In this contribution we provide extension of our previous theoretical studies using beam propagation model incorporating laser rate equations. The effect of bending and twisting on signal amplification in the double-clad fiber is analyzed for different input signal powers, and moreover, pump field modal spectra are evaluated. The results show that in correspondence with pump absorption efficiency the gain of the amplifier is enhanced under the conditions of simultaneously bent and twisted fiber. The key to understand the effect of bending and twisting on pump absorption efficiency consists in modal spectra of pump field propagating in the first clad of the double clad fiber. Three cases of straight, bent only, and simultaneously bent and twisted fiber are compared. The comparison shows that bending causes increase of the spectral range of propagating modes, but does not bring about mode-mixing. Substantial mode-mixing is established only in simultaneously bent and twisted fiber.

In this paper we report the generation of flat top optical spectrum using an arbitrary waveform generator to increase the SBS threshold in high power optical fiber amplifiers. The optical spectrum consists of a number of discrete spectral lines, ranging from 16 to 380, within the bandwidth of 2GHz, corresponding to line spacing between 133 MHz and 5 MHz. These discrete spectral lines correspond to a PRBS pattern of n = 4 to n = 8. The SBS threshold and coherence properties of the flat top spectrum are measured and compared to that of the filtered PRBS in a kilowatt class fiber amplifier. It is experimentally demonstrated that for large frequency line spacing, the flat top spectrum significantly outperforms the corresponding filtered PRBS, but as the line spacing is decreased to less than the Brillouin bandwidth, the two modulation waveforms have similar enhancement factors in the SBS threshold due to the enhanced crosstalk between neighboring frequency components.

The noise characteristics of high-power fiber lasers, unlike those of other solid-state lasers such as thin-disks, have not been systematically studied up to now. However, novel applications for high-power fiber laser systems, such as attosecond pulse generation, put stringent limits to the maximum noise level of these sources. Therefore, in order to address these applications, a detailed knowledge and understanding of the characteristics of noise and its behavior in a fiber laser system is required. In this work we have carried out a systematic study of the propagation of the relative intensity noise (RIN) along the amplification chain of a state-of-the-art high-power fiber laser system. The most striking feature of these measurements is that the RIN level is progressively attenuated after each amplification stage. In order to understand this unexpected behavior, we have simulated the transfer function of the RIN in a fiber amplification stage (~80μm core) as a function of the seed power and the frequency. Our simulation model shows that this damping of the amplitude noise is related to saturation. Additionally, we show, for the first time to the best of our knowledge, that the fiber design (e.g. core size, glass composition, doping geometry) can be modified to optimize the noise characteristics of high-power fiber laser systems.

Hybrid microstructured fibers, utilizing both air holes and high index cladding structures, provide important advantages over conventional fiber including robust fundamental mode operation with large core diameters (>30μm) and spectral filtering (i.e. amplified spontaneous emission and Raman suppression). This work investigates the capabilities of a hybrid fiber designed to suppress stimulated Brillouin scattering (SBS) and modal instability (MI) by characterizing these effects in a counter-pumped amplifier configuration as well as interrogating SBS using a pump-probe Brillouin gain spectrum (BGS) diagnostic suite. The fiber has a 35 μm annularly gain tailored core, the center doped with Yb and the second annulus comprised of un-doped fused silica, designed to optimize gain in the fundamental mode while limiting gain to higher order modes. A narrow-linewidth seed was amplified to an MI-limited 820 W, with near-diffraction-limited beam quality, an effective linewidth ~ 1 GHz, and a pump conversion efficiency of 78%. Via a BGS pump-probe measurement system a high resolution spectra and corresponding gain coefficient were obtained. The primary gain peak, corresponding to the Yb doped region of the core, occurred at 15.9 GHz and had a gain coefficient of 1.92×10-11 m/W. A much weaker BGS response, due to the pure silica annulus, occurred at 16.3 GHz. This result demonstrates the feasibility of power scaling hybrid microstructured fiber amplifiers

A 3 kW single stage all-fiber Yb-doped single-mode fiber laser with bi-directional pumping configuration has been demonstrated. Our newly developed high-power LD modules are employed for a high available pump power of 4.9 kW. The length of the delivery fiber is 20 m which is long enough to be used in most of laser processing machines. An output power of 3 kW was achieved at a pump power of 4.23 kW. The slope efficiency was 70%. SRS was able to be suppressed at the same output power by increasing ratio of backward pump power. The SRS level was improved by 5dB when 57% backward pump ratio was adopted compared with the case of 50%. SRS was 35dB below the laser power at the output power of 3 kW even with a 20-m delivery fiber. The M-squared factor was 1.3. Single-mode beam quality was obtained. To evaluate practical utility of the 3 kW single-mode fiber laser, a Bead-on-Plate (BoP) test onto a pure copper plate was executed. The BoP test onto a copper plate was made without stopping or damaging the laser system. That indicates our high power single-mode fiber lasers can be used practically in processing of materials with high reflectivity and high thermal conductivity.

Fiber amplifiers are representing one of the most promising solid state laser concepts, due to the compact setup size, a simple thermal management and furthermore excellent beam quality. In this contribution, we report on the latest results from a low-NA, large mode area single mode fiber with a single mode output power beyond 4 kW without any indication of mode instabilities or nonlinear effects and high slope efficiency. Furthermore, we quantify the influence of the bending diameter of our manufactured low NA fiber on the average core loss by an OFDR measurement and determine the optimal bending diameter in comparison to a second fiber with a slightly changed NA. The fibers used in the experiments were fabricated by MCVD technology combined with the solution doping technique. The investigation indicates the limitation of the step index fiber design and its influence on the use in high power fiber amplifiers. We demonstrate, that even a slightly change in the core NA crucially influences the minimum bending diameter of the fiber and has to be taken into account in applications. The measured output power represents to the best of our knowledge the highest single mode output power of an amplifier fiber ever reported on.

Transverse mode instability (TMI) has been recognized as a major limit to average power scaling of single-mode fiber laser besides the optical nonlinear effects. One key to mitigate TMI is to suppress the higher-order modes (HOMs) propagation in the optical fiber. By implementing additional cores in the optical fiber cladding, HOMs can be resonantly coupled from the main core to the surrounding cladding cores, leading to better HOMs suppression. Here, we demonstrate an Yb-doped multiple-cladding-resonant all-solid photonic bandgap fiber with a ~60μm diameter core for high power fiber lasers. The fiber has a multiple-cladding-resonant design in order to provide better HOMs suppression. Maximum laser power of 910w is achieved for a direct diode-pumped fiber laser without TMI with a 9m long fiber at 60cm coil diameter, breaking the TMI threshold of 800w that has been observed in large-mode-area PCFs with ~40μm core. This result is limited by fiber end burning due to the un-optimized thermal management. Later experiment demonstrates maximum laser power of 1050w with 90% lasing efficiency versus absorbed pump power in a 8m long fiber coiled at 80cm diameter, limited by the pump source. However, the fiber bending condition needs to be optimized in order to produce a better laser beam quality.

The high performance of fibre lasers is largely due to the outstanding characteristics of fibres as an active medium. However, there is a need to overcome some limits at high optical powers which are imposed by the fibre design. We report on a design and fabrication of a stimulated Raman scattering (SRS) filtering fibre for high average or high peak optical power delivery applications. The fibre geometry is based on the series of circularly arranged high index rod resonators embedded in the silica cladding. The operation principle relies on the resonant coupling of the core and resonators modes. The fabricated fibre demonstrated wide transmission window and filtering of SRS from the output spectra (with the extinction which exceeds 20dB at the Raman Stokes wavelength), robustness for bending and high output beam quality. The fibre has been tested as a beam delivery fibre of a commercial pulsed fibre laser system in order to identify filtering performance and its limitations.

Simple method to increase stimulated Brillouin scattering (SBS) threshold in MCVD fiber based on design with few concentric layers having different compound has been proposed. Two sets of fibers with core consisting of three layers with different alumina and germania concentrations have been fabricated. First set of fibers was designed for Raman amplifiers and had a relatively small mode field area of 23-28 μm2. The second set of fibers was designed for high peak power pulse delivery and had mode area of 225-325 μm2. SBS suppression (as compared to the Ge-doped fibers) was estimated from SBS gain spectra and direct observation of SBS threshold to be more than 6 dB and 3.3 dB for the first and the second set of fibers.

A polarization-maintaining Yb-doped large mode area fiber with depressed-index inner cladding layer and confinement of rare-earth dopants has been drawn as a long tapered fiber. The larger end features a core/clad diameter of 56/400 μm and core NA ~ 0.07, thus leading to an effective mode area over 1000 μm2. The fiber was tested up to 100 W average power, with near diffraction-limited output as the beam quality M2 was measured < 1.2. As effective single-mode guidance is enforced in the first section due to enhanced bending loss, subsequent adiabatic transition of the mode field in the taper section preserves single-mode amplification towards the larger end of the fiber.

We report on the impact of the cross-sectional shape of the inner cladding of double-clad fiber on pump absorption using a Beam Propagation Method based on azimuthal harmonics expansions and the so-called "superformula" to describe complex shapes. The impact of shaped cladding and of the fiber layout on pump absorption of double-clad fiber is studied. We observe that for nearly circular cross-section fibers the pump absorption can be enhanced to match that of fiber with strong azimuthal asymmetry, through optimised fiber layout that maximizes pump mixing.

We designed the large mode area photonic crystal fiber structure. We calculated the effective index, and dispersion of fundamental mode using finite difference time domain method. Numerical results show that fiber structure strips higher order modes from the photonic crystal fiber and retains only fundamental mode with 10cm bend. We have shown the results of two types of photonic crystal fibers. The fiber shows negative dispersion from 1.860 μm to 1.996 μm wavelength. We fabricated the photonic crystal fiber structure and analyzed the fabrication difficulties of the fiber design.

Recent progress in our work on the development of three micron class dysprosium-doped ZBLAN fiber lasers will be presented. Of particular note is the achievement of 51% slope efficiency which to our knowledge represents a record for all 3 micron class fiber lasers. This result is obtained for an in-band pumping scheme which also allowed for demonstration of continuous tuning over a range of 400nm with an extreme emission wavelength of 3.35 microns.

We demonstrate a 74 mol % GeO2 doped fiber for mid-infrared supercontinuum generation. Experiments ensure a highest output power for a broadest spectrum from 700nm to 3200nm from this fiber, while being pumped by a broadband 4 stage Erbium fiber based MOPA. The effect of repetition rate of pump source and length of Germania-doped fiber has also been investigated.

Further, Germania doped fiber has been pumped by conventional Silica based photonic crystal fiber supercontinuum source. At low power, a considerable broadening of 200-300nm was observed. Further broadening of spectrum was limited due to limited power of pump source. Our investigations reveal the unexploited potential of Germania doped fiber for mid-infrared supercontinuum generation. This measurement ensures a possibility of Germania based photonic crystal fiber or a step-index fiber supercontinuum source for high power ultra-broad band emission being pumped a 1060nm or a 1550nm laser source. To the best of our knowledge, this is the record power, ultra-broadband, and all-fiberized SC light source based on Silica and Germania fiber ever demonstrated to the date.

Coherent beam combining (CBC) by active phase control could be useful for power scaling fiber-laser-pumped optical frequency converters like optical parametric oscillators (OPOs). However, a phase modulator operating at the frequency-converted wavelength would be needed, which is a non-standard component.

Fortunately, nonlinear conversion processes rely on a phase-matching condition, correlating not only the wave-vectors of the coupled waves, but also their phases. It is therefore possible to control the phase indirectly, using more standard phase modulators.

Feasibility of this technique was previously demonstrated for second harmonic generators (SHG). Controlling the phase of the fundamental wave, excellent harmonic wave combining efficiency was achieved in both cases of phase matching and quasi phase matching, with lower than λ/30 residual phase error.

In this paper, coherent combining of difference frequency generators (DFG) is experimentally tested. Even if DFG is more challenging than SHG as it implies handling three waves instead of two, phase control of the sole 1-μm pump waves is sufficient to combine the 3.4-μm waves generated.

The mid-infrared DFG crystals are pumped and signal-seeded with standard all-fiber sources at 1 μm and 1.5 μm respectively. Phase control is performed with an electro-optic phase modulator which is a standard all-fiber component operating at 1 μm.

We demonstrate a passively mode-locked holmium-praseodymium co-doped ring fiber laser that produces an estimated 950 fs pulsewidth and peak power of 4.3 kW at a pulse repetition rate of 74 MHz. The measured center wavelength was 2.86 µm which overlaps more strongly with liquid water whilst better avoiding atmospheric water vapor which overlaps more strongly with previously reported ultrafast Er3+ fiber lasers operating at 2.8. Thus the present system should display better long term stability compared to the Er3+-based system and at the same time, be a more practical tool for interaction with biological tissues.
The laser was constructed using a 1.2 m long double-clad fluoride fiber doped with Ho3+ and Pr3+ ions and arranged into a unidirectional ring resonator that was resistant to instabilities associated with back reflections. Two semiconductor 1150 nm laser diodes with the maximum combined output of 7.5 W were used to pump the fiber. Mode-locking was achieved using the combination of two techniques: sub-picosecond pulses were produced by nonlinear polarization evolution after longer pulses were initially obtained using an in-cavity GaAs saturable absorber having a modulation depth of 90% and a relaxation time of 10 ps. A standard arrangement employing two waveplates and an optical isolator was introduced into the resonator to carry out nonlinear polarization rotation. The average power of the mode-locked laser reached 350 mW after the 50% outcoupling mirror. The RF signal-to-noise ratio reached 67 dB for the first peak at the resolution bandwidth of 10 kHz.

The output of solid core fiber lasers is constrained in the mid-infrared due to the absorption properties of silica. Optically pumped gas lasers can reach the mid-infrared but require long path lengths for interaction between the pump light and gain medium. Optically pumped gas lasers where the gain medium is contained in a hollow-core optical fiber may provide a robust and compact platform that combines advantages of fiber and optically-pumped gas lasers. Experimental demonstrations of gas-filled-fiber lasers have been reported. The energy output of a molecular gas laser operating in a hollow-core optical fiber is computationally modeled using rate equations. The rate equations include terms for various physical processes including molecular self-collisions, molecular collisions with the fiber walls, and fiber attenuation. The rate equations are solved for a time-dependent, one-dimensional fiber model with an acetylene gain medium that lases along rotation-vibrational transitions. The energy output and losses are computed for multiple configurations. Model correspondence with reported experiments is shown. The computed energy losses due to backwards propagating light, fiber losses, and molecular collisions are applied to pulsed, continuous wave, and synchronously pumped gas lasers operating in hollow-core optical fibers. Energy losses due to molecular collisions are used to estimate heating in the gain medium.

We present an entirely fiber based laser source for non-linear imaging with a novel approach for multi-color excitation. The high power output of an actively modulated and amplified picosecond fiber laser at 1064 nm is shifted to longer wavelengths by a combination of four-wave mixing and stimulated Raman scattering. By combining different fiber types and lengths, we control the non-linear wavelength conversion in the delivery fiber itself and can switch between 1064 nm, 1122 nm, and 1186 nm on-the-fly by tuning the pump power of the fiber amplifier and modulate the seed diodes. This is a promising way to enhance the applicability of short pulsed laser diodes for bio-molecular non-linear imaging by reducing the spectral limitations of such sources. In comparison to our previous work [1, 2], we show for the first time two-photon imaging with the shifted wavelengths and we demonstrate pulse-to-pulse switching between the different wavelengths without changing the configuration.

Fiber lasers provide the perfect basis to develop broadly tunable lasers with high efficiency, excellent beam quality and user-friendly operation as they are increasingly demanded by applications in biophotonics and spectroscopy. Recently, a novel tuning scheme has been presented using fiber Bragg grating (FBG) arrays as fiber-integrated spectral filters containing many standard FBGs with different feedback wavelengths. Based on the discrete spectral sampling, these reflective filters uniquely enable tailored tuning ranges and broad bandwidths to be implemented into fiber lasers. Even though the first implementation of FBG arrays in pulsed tunable lasers based on a sigma ring resonators works with good emission properties, the laser wavelength is tuned by a changing repetition rate, which causes problems with applications in synchronized environments.
In this work, we present a modified resonator scheme to maintain a constant repetition rate over the tuning range and still benefit from the advantages of FBG arrays as filters. With a theta ring cavity and two counter propagating filter passes, the distributed feedback of the FBG array is compensated resulting in a constant pulse round trip time for each filter wavelength. Together with an adapted gating scheme controlling the emission wavelength with a modulator, the tuning principle has been realized based on a Ytterbium-doped fiber laser. We present first experimental results demonstrating a tuning range of 25nm, high signal contrast and pulse durations of about 10ns. With the prospect of tailored tuning ranges, this pulsed fiber-integrated laser may be the basis to tackle challenging applications in spectroscopy.

Among other modern imaging techniques, stimulated Raman Scattering (SRS) requires an extremely quiet, widely wavelength tunable laser, which, up to now, is unheard of in fiber laser systems. We present a compact all-fiber laser system, which features an optical parametric oscillator (OPO) based on degenerate four-wave mixing (FWM) in an endlessly single-mode photonic-crystal fiber. We employ an all-fiber frequency and repetition rate tunable laser in order to enable wideband conversion in the linear OPO cavity arrangement, the signal and idler radiation can be tuned between 764 and 960 nm and 1164 and 1552 nm at 9.5 MHz. Thus, all biochemically relevant Raman shifts between 922 and 3322 cm-1 may be addressed in combination with a secondary output, which is tunable between 1024 and 1052 nm. This ultra-low noise output emits synchronized pulses with twice the repetition rate to enable SRS imaging. We measure the relative intensity noise of this output beam at 9.5 MHz to be between -145 and -148 dBc, which is low enough to enable high-speed SRS imaging with a good signal-to-noise ratio. The laser system is computer controlled to access a certain energy differences within one second. Combining FWM based conversion, with all-fiber Yb-based fiber lasers enables the construction of the first automated, turn-key and widely tunable fiber laser. This laser concept could be the missing piece to establish CRS imaging as a reliable guiding tool for clinical diagnostics and surgical guidance.

In this communication, the authors report on the first high peak-power emission obtained using a solid non-filamented core fully-aperiodic large pitch fiber manufactured by the REPUSIL method which is based on the sintering and vitrification of micrometric doped silica powders. Using a simple amplifier stage based on a 75 cm long piece of a fullyaperiodic large pitch fiber with a fiber core of 50 μm, an average output power of 95 W was achieved with an available pump power of 175 W, corresponding to an optical-to-optical efficiency of 54 %. The peak power reaches about 35 kW for pulse duration of 200 ps at a repetition rate of 13.5 MHz. A recent evolution of our set-up using a seeder delivering an average power of 4 W at 1 MHz with a pulse duration of 50 ps led to the emission of 71.4W in average power corresponding to a peak power of 1.42 MW. These results present the first demonstration of high average and high peak power in pulsed regime for these fibers.

High power short pulse fiber lasers are applied in industry for many ablation processes or various surface treatments, and there is a huge demand for such lasers but with higher average power, higher pulse energy and higher peak power. This contribution presents a high peak- and average- power fiber laser with selectable pulse durations between 10 ns and 100 ns, where more than 150 mJ pulse energy has been achieved at a repetition rate of 10 kHz. In addition, for a laser pulse with 30 ns pulse duration a maximum peak power of more than 3.5 MW at more than 1 kW average output power have been demonstrated. These results could be achieved by applying extra-large mode area (XLMA) gain fibers (fiber core <100 μm) in the fiber amplifiers and using pulse shape capabilities of the seed laser, only. Stable and safe operation of the fiber laser have been shown with power densities up to 3 GW/cm² in the gain fiber. In order to protect the fiber laser to be affected by back reflections from the workpiece, a newly designed optical isolator with more than 30 dB isolation has been implemented.

The powerful picosecond master oscillator – power amplifier (MOPA) with double clad ytterbium doped tapered fiber as a buster amplifier has been demonstrated in the presented paper. The developed MOPA has 60ps pulses with 0.3mJ pulse energy and 5MW peak power.

We present a simple way to achieve and optimize hundreds of kW peak power pulsed output using a monolithic amplifier chain based on solid core double cladding fiber tightly packaged. A fiber pigtailed current driven diode is used to produce nanosecond pulses at 1064 nm. We present how to optimize the use of Fabry-Perot versus DFB type diode along with the proper wavelength locking using a fiber Bragg grating. The optimization of the two pre-amplifiers with respect to the pump wavelength and Yb inversions is presented. We explain how to manage ASE using core and cladding pumping and by using single pass and double pass amplifier. ASE rejection within the Yb fiber itself and with the use of bandpass filter is discussed. Maximizing the amplifier conversion efficiency with regards to the fiber parameters, glass matrix and signal wavelength is described in details. We present how to achieve high peak power at the power amplifier stage using large core/cladding diameter ratio highly doped Yb fibers pumped at 975 nm. The effect of pump bleaching on the effective Yb fiber length is analyzed carefully. We demonstrate that counter-pumping brings little advantage in very short length amplifier. Dealing with the self-pulsation limit of stimulated Brillouin scattering is presented with the adjustment of the seed pulsewidth and linewidth. Future prospects for doubling the output peak power are discussed.

We demonstrate an all fiber picosecond laser with megawatt-level peak power based on highly ytterbium-doped phosphate fiber. The phosphate fiber used in the power amplifier stage has the cladding absorption 50 dB/m at 976 nm. The length of the gain fiber in the main amplifier is only 34 cm, which will effectively reduce the nonlinear effect during the process of picosecond pulse amplification. The core diameter of the gain fiber is 25 μm with numerical aperture of 0.04, which is helpful to obtain high beam quality laser. The pulse energy of the seed is 0.2 μJ at repetition rate of 25 kHz, which is amplified to 21 μJ with the pulse width of 20 ps and the peak power is 1.05 MW. High beam quality is also demonstrated with M2 factor measured at highest pulse energy of 1.4 in the both X and Y directions. This kind of laser source with high peak power and high beam quality has a wide range of applications in the field of material processing.